There is essentially no limit to the sources from which hydrogen can be obtained. Furthermore, it produces no harmful waste materials when burnt, and is fairly easy to transport and store. Because of these advantages, hydrogen is now considered as one of the new energy substitutes for fossil fuels such as petroleum and coal. Accordingly, there is a great demand for the development of safe and economical technology for industrial transportation and storage of hydrogen. Such technology must of course be adapted to the future use of hydrogen in large quantities and for various purposes.
The most promising methods that have been proposed for transporting and storing hydrogen on an industrial scale is to solidify hydrogen in the form of a metal hydride. According to one experimental method, hydrogen produced by electrolysis of water with excess electric power is stored in the form of a solid metal hydride until it is recovered for use as a source of energy. Rare earth transition metals (e.g. LaNi.sub.5) and Ti-Fe have been proposed as substances to be chemically combined with hydrogen.
However, these metallic compounds have the following defects: (1) their hydrides break into fine particles as a result of repeated cycles of storing and releasing hydrogen; (2) they comprise expensive metals or alloys whose supply is rather limited; (3) these metallic compounds are heavy and not easy to transport and handle; and (4) the hydride must be stored at two or more atmospheres to prevent evaporation of the hydrogen.